Chop Shop works but no clipping / drive

MaxIokua

MaxIokua

New member
Hello, I'm new on here, and new to pedal building (just doing kits), so hopefully I'm asking this in the right place...
I just built a chop shop, calibrated the J201s to ~6VDC, and tried it out. It works, but there is such little drive (actual clipping) that is sounds like a clean boost and nothing else. The signal doesn't break up at all. Volume at 50%, Sag at 0 (fully CCW), and drive at 100% (fully CW).

Any ideas on what to look for would be greatly appreciated. I understand that J201s are not all created equal, could they be that far out of spec as not to function even mildly as intended? Or am I missing something. The parts came from musikding so I would expect that the J201s are decent quality. I have already performed general troubleshooting duty (ie, reflowing solder, rechecking components, wire connections, power supply etc)
Thanks!
ChopShopGutShot.jpg
 
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Interesting! That would explain a lot. Now to find real J201s...! I started a cursory search when the drive wasn't behaving as expected, it looks like they are hard to find. Is the MMBFJ201 considered comparable in this application?
 
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temol said:
From the original J201 I would expect a Vp value in the range of -0.6V to -1.0V.

How to measure Vp.

Half my J201s are bad?

Long story short, it looks like half my through-hole J201s are bad. Short story long: I was troubleshooting my in-progress Low Tide build, as it had farty, distorted output, which pointed to one of the JFETs. I assumed I had done a poor job soldering the surface mount 2Sk208, but my audio probe...
forum.pedalpcb.com forum.pedalpcb.com

The fact that you can set 4V, 6V or any other voltage does not matter much. The same can be done here for J201, 2N5457, 2N5458 - thanks to the fact that you have trimmers that allow you to change the resistance on the drain side. What is more important is what transistor (what parameters) was used.
Click to expand...
I must be doing something wrong here. Following the breadboard layout from the link, I am getting ~6VDC Vp, and ~6mA Idss. Not .6, but full 6. I have double checked the procedure and measurements. I get similar results when using 1k/1 meg resistor method for testing Vp and Idss which I found elsewhere ("simple circuit to test JFET characteristics").
 
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Thank you again, @temol - your diagram was much easier to comprehend....I'm now getting numbers that make more sense. 1.148 volts on Q1, 1.132 volts on Q2. Power supply was a 9v battery. So the Vp is a bit high for the purpose but not too crazy. I tried to test Idss by shorting G and S and inserting the meter between D and the battery anode, and I am getting ~ 902 uA on Q1 and ~880 on Q2 which also seems slightly out of the ideal range. So maybe these are in fact J201s, just slightly higher readings than are considered ideal for this particular build.
 
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My opinion - Vp is a bit high as for the J201. I have some MMBF5457 transistors with Vp -1.2V.

Here's some Barbershop simulation results from ltspice (from the output). Note the Vp next to the JFET name.

1705777378897.png
 
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Regardless, the drive doesn't sound like it should. To my ear the best (richest/fullest) sound I can get out of it is with Q1 and 2 at ~3.9V, but at this setting there is close to no clipping at max drive, and the sag control does almost nothing. If I adjust Q1 and 2 to the point where they will clip (2V or less), the pedal sounds sharp and anemic, which is not how it should be, at least when comparing it to an actual Barbershop. I have rechecked to make sure all resistors etc are the right values in the right place. I did notice that I must have an older version with fewer parts count than the one currently in the PedalPCB catalog, which may be part of the issue. But I am going to await the arrival of some different J201s (which will take a while as I'm in a remote area - expected February). Meantime, I have a usable clean boost but that's it.
 
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I received the J201s I ordered much earlier than expected. Tested and selected Q1 as Vp=-.984V Idss=777uA and Q2 as Vp=-.865V Idss=588uA, trying to match Gray Bench's findings as closely as possible. Unfortunately this was not the magic bullet, although it did improve the anemia (lack of low end, thinness) and the lack of sag control slightly. Bias still below the 5-6.5V that most report. By ear, the bias remained similar to the original J201s, around 3.9V. Replacing the 2N2222A made no difference. After comparing Gray Bench's schematic with my pcb build, I have to conclude that they are 2 slightly different circuits (not just the addition of tone switch!) and this is the reason the pedal doesn't quite sound as it should. I plan to build another following Gray Bench's schematic in order to find out for sure. This has been a really good learning experience and hopefully will help out someone else who is wondering how much of a difference 'real' J201s would make.
Some photos to compare the markings on the old J201s to the new...

Kit J201:
old J201s.jpg

Vint'Elec J201:
VintElec J201.jpg
 
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MaxIokua said:
After comparing Gray Bench's schematic with my pcb build, I have to conclude that they are 2 slightly different circuits (not just the addition of tone switch!) and this is the reason the pedal doesn't quite sound as it should.
Click to expand...

What you have is the older V1 circuit.

The Gray Bench schematic is for the newer V2 with tone switch.

This one:
www.pedalpcb.com

Chop Shop Overdrive - PedalPCB.com

Compare to Fairfield Circuitry Barbershop
www.pedalpcb.com www.pedalpcb.com

 
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Thanks @Robert, I had meant to get V2 but was sent V1 by Musikding. Many chop shop builds I scoured on the forum seem to be V1s so I didn't catch on at first when I was troubleshooting.
Just wanted to add that I have swapped out Q1 for Vp -.865/Idss 588 and Q2 Vp .726/Idss 389 and, after playing for a few hours, can say I'm getting marginally better and very usable results. Lower values sound better, to my ears.

Looking forward to building a V2 to compare!
 
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